1. Field of the Invention
The present invention relates to a touch sensing system.
2. Discussion of the Related Art
User interface (UI) is configured so that users are able to communicate with various electronic devices and thus can easily and comfortably control the electronic devices as they desire. Examples of the user interface include a keypad, a keyboard, a mouse, an on-screen display (OSD), and a remote controller having an infrared communication function or a radio frequency (RF) communication function. User interface technology has continuously expanded to increase user's sensibility and handling convenience. The user interface has been recently developed to include touch UI, voice recognition UI, 3D UI, etc.
The touch UI has been indispensably adopted in portable information appliances. The touch UI is implemented through a method for forming a touch screen on the screen of a display device.
As shown in FIG. 1, a mutual capacitive touch screen includes Tx lines Tx1 to Tx4, Rx lines Rx1 to Rx4 perpendicular to the Tx lines Tx1 to Tx4, and touch sensors formed between the Tx lines Tx1 to Tx4 and the Rx lines Rx1 to Rx4. Each touch sensor includes a mutual capacitance Cm. A sensing circuit supplies a driving signal to the Tx lines Tx1 to Tx4 and receives a touch sensor signal synchronized with the driving signal through the Rx lines Rx1 to Rx4. The sensing circuit senses a change amount of charges of the touch sensor and analyzes the change amount of charges. Hence, the sensing circuit decides whether or not there is a touch input and finds out a position of the touch input when there is the touch input. The sensing circuit may be integrated into touch sensing integrated circuits (ICs) and may be connected to the Tx lines Tx1 to Tx4 and the Rx lines Rx1 to Rx4.
Differential amplifiers 11 to 14 may be connected to the Rx lines Rx1 to Rx4. The sensing circuit may receive a signal amplified by the differential amplifiers 11 to 14, each of which is connected to the two adjacent Rx lines. An output terminal of each of the differential amplifiers 11 to 14 is connected to an inverting input terminal (−) via a capacitor C. Each of the differential amplifiers 11 to 14 amplifies a difference between an ith sensor signal input to the inverting input terminal (−) and an (i+1)th sensor signal input to a non-inverting input terminal (+) and outputs ith sensor signals S1 to S4, where ‘i’ is a positive integer. As shown in FIG. 2, the differential amplifiers 11 to 14 amplify a difference between the signals received through the adjacent Rx lines and further increase signal components than a noise, thereby improving a signal-to-noise ratio (SNR).
In a method for obtaining the sensor signal through the differential amplifiers 11 to 14, (N−1) sensor signals may be obtained when N Rx lines are used, where N is a positive integer equal to or greater than 2. There is a method for applying a virtual dummy signal to a non-inverting input terminal of the differential amplifier connected to a first receiving channel (hereinafter referred to as “Rx channel”) or a last Rx channel of the sensing circuit, so as to obtain the N sensor signals. However, the method has the problem, in which the signal-to-noise ratio of some sensor signals is reduced. In particular, the signal-to-noise ratio of one of edge Rx channels (i.e., the first Rx channel and the last Rx channel) positioned at both ends among the Rx channels of the sensing circuit is reduced.
In an example shown in FIG. 1, a dummy signal D0 is input to the non-inverting input terminal (+) of the fourth differential amplifier 14. The fourth differential amplifier 14 outputs a signal obtained by amplifying a difference between a fourth sensor signal and the dummy signal D0 as a fourth sensor signal. The dummy signal D0 does not have a noise, unlike the sensor signal. Thus, when the difference between the fourth sensor signal and the dummy signal D0 input to the fourth differential amplifier 14 is amplified, the noise is amplified. As a result, an improvement effect of the signal-to-noise ratio of the signal output from the fourth differential amplifier 14 is less than the other differential amplifiers 11 to 13.
When the size and a resolution of the touch screen increase, the number of transmitting channels (hereinafter referred to as “Tx channels”) and the number of Rx channels of the touch screen increase. Thus, when the size and the resolution of the touch screen increase, two or more ICs need to be connected to the touch screen.
U.S. Pat. No. 8,350,824 B2 disclosed a method for connecting two ICs to a large-sized touch screen and obtaining sensor data (hereinafter referred to as “boundary data”) at a boundary between the two ICs. U.S. Pat. No. 8,350,824 B2 proposed a method for low-pass filtering the boundary data between the ICs and data adjacent to the boundary data and generating the boundary data using a low-pass filtering value as an average value, so as to obtain the boundary data. However, such a sensing method has to compare the data adjacent to the boundary data and calculate the average value of the adjacent data, so as to obtain the boundary data. Hence, a processing amount of data increases, and data processing time increases. Further, when there is a large output deviation between the ICs, the accuracy of data may be reduced.